Phased antenna arrays typically comprise a group of antennas in which the relative phases of the respective signals feeding the antennas are varied. The phases of the antennas may be arranged in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. If the phases supplied to the respective antennas can be varied, then the direction of a beam transmitted by the array may be steered.
The beam forming characteristics of phased array antennas make them attractive in the fields of radar, wireless communications, RFID etc.
There are various ways of controlling the phase supplied to each antenna. One popular technique is to use micro-electromechanical systems (MEMs) to vary the phase delay on a capacitor diode loaded transmission line. The general arrangement is shown in FIG. 1. An oscillator 1 provides a radio frequency signal, which is split by power divider 10 into two signals, each of which is passed through a respective phase shifter 20, 30. The phase shifters comprise a capacitor diode loaded transmission line having a phase delay, which can be adjusted by varying the arrangement of diodes and capacitors with a MEMs system. Each phase shifter shifts the phase of the signal passing through it by a respective amount and outputs a phase shifted RF signal 40, 50.
This arrangement has several disadvantages. Firstly, the signal energy from the oscillator is put through a power divider and thus the output signal 40, 50 delivered to each antenna decreases as the number of antennas increases. Furthermore power dividers tend to be complicated and lossy and thus add to the expense and further decrease the power delivered. Secondly, as the output signal itself passes through the phase shifters, insertion loss and noise are introduced. Thus, amplifiers are required to compensate the power loss and this makes the system complex and expensive.
Another solution is to use a DDS (Direct Digital Synthesizer) to produce signals having the desired phase directly, at a lower frequency, and then up-converting to a higher frequency.